I'm approximating land values. You're correct that a developer could reap much more than what the raw land value would bring in if the land could be subdivided into many homes.

But you can't back into a value for Mar-A-Lago that way - because so much of the value of a piece of property is derived from the use of the property, not merely the size of the lot. If you take a 4-acre single family luxury residence of $27 million and add another four acres - but aren't allowed to increase the number or size of the house - it's not going to now be worth $54 million. It will be worth more than before, but not a whole lot more. Because adding empty green space to a residential building doesn't really make the property all that more valuable if you can't build anything on the green space.

If you actually look at what Mar-a-Lago is, it has a very wonderful big old mansion (55 BR, 33BA) on 17 acres of land. But the land can't be subdivided, the mansion can't be expanded (it's historic), it can't be operated as a hotel (and it's too small and not built to be and it's not zoned for it), and it has relatively modest amenities. Most of the land area is just open lawn or parking. It's a long deep property - meaning it has relatively small waterfront relative to the overall size.

It's just insupportable to claim that it was worth $400-600 million dollars.